Solid polymer electrolyte has been of great interest due to its possible application in high energy density batteries. Using the solid polymer electrolyte to fabricate a totally solid battery cell will improve the reliability of the battery cell due to leakage of the contents in the cell being avoided. Furthermore, a thin cell and thus a multiple-cell battery will come true.
The characteristic properties of a solid polymer electrolyte generally include (a) a high ionic conductivity without involving any electron conductivity; (b) good film-forming properties where a thin film can be formed; and (c) good flexibility.
The solid polymer electrolyte is composed substantially of a flexible matrix polymer and an alkali metal salt, where a complex structure is formed when they are blended. The alkali metal salt in the matrix polymer selectively ionizes the amorphous sites in the matrix polymer and moves by diffusion along the electric field in the matrix, thereby achieving the ionic conduction while interacts with the coordinating atoms in the polymer. The resulting complex polymer is capable of forming a flexible thin film that is imparted with good mechanical properties owing to the flexibility inherent to polymer. The thin film consisting of the complex polymer can be appropriately adapted for the volumetric variation caused by the ion-electron exchange reaction between the electrode and the solid polymer electrolyte. For these reasons, the solid polymer electrolyte material is particularly suitable for high energy density battery cells, particularly, thin cells.
In general, the matrix polymer is crystalline in nature, such as poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVdF), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA) . . . etc. Therefore, the ionic conductivity of the complex polymer film consisting of the matrix polymer and the alkali metal salt decreases abruptly at the temperatures lower than the melting point of the matrix polymer. For instance, the complex polymer film consisting of PEO having a molecular weight of about 10,000 and alkali metal salt has an ionic conductivity of 10.sup.-3.about.10.sup.-4 S/cm at temperatures of 100.degree. C., or higher. However, its ionic conductivity decreases to a very small value of not higher than 10.sup.-7.about.10.sup.-8 S/cm at room temperatures. The solid polymer electrolyte does not meet the commercial requirements at ambient temperatures.
Various attempts have been made to improve the ionic conductivity of solid polymer electrolytes by developing new polymeric materials such as polymer containing alkyl quaternary ammonium salt (U.S. Pat. No. 5,643,490) and polybenzimidazole doped with H.sub.3 PO.sub.3 (U.S. Pat. No. 5,688,613). Even by such means, however, the improvement in the ionic conductivity of the complex polymer film is not yet satisfactory. Furthermore, using the acid as a conductor may lead to the leakage of the cell contents. Other suggestions include the use of a large amount of plasticizers addition to polymer electrolytes to form "wet" polymer or "gel electrolyte" (U.S. Pat. Nos. 5,581,394; 5,705,084; 5,645,960; 5,731,104; 5,586,001), which procedure does improve ambient temperature conductivity but this is done at the expense of mechanical properties.
U.S. Pat. No. 5,609,974 discloses a solid polymer electrolyte (SPE) which is prepared by polymerization of three selected monomers together with a lithium salt and plasticizers. One of the monomer is a compound having two acryloyl functionalities which serves as a crosslinking agent for example a diacrylate. Another selected monomer is a compound having one acryloyl or allyl functionality and also contains groups with high polarity such as carbonate or a cyano group. Another of the selected monomers is a compound having one acryloyl functionality and an oligo (oxyethylene) group. The monomer which includes the carbonate or a cyano group serves to enhance the conductivity since either one of these groups provides an appreciable acceptor number which quantifies the possibility for anion solvation thus making the electrolyte salt more conductive. The monomer having the oligo (oxyethylene) side chain provides the resulting polymer with flexibility and free volume for the movement of ions, and also provides the resulting solid polymer electrolyte with compatibility with plasticizers. These solid polymer electrolytes have an ionic conductivity ranging from 10.sup.-4.about.10.sup.-5 S/cm which is slightly better than that of the solid polymer electrolytes consisting of PEO and an alkali metal salt, but is not satisfactory in practical application. Furthermore, the mechanical properties of these solid polymer electrolytes are adversely affected, so that their film-forming properties and flexibility are also not satisfactory.
Attentions have been paid to lithium metal secondary cells for use as high-capacitance cells. There is a strong demand for developments of a solid polymer electrolyte that is high in ionic conductivity and has excellent film-forming properties, flexibility and mechanical properties.
It is apparent from the foregoing that the prior art is still not able to fabricate a solid polymer electrolyte in the form of a film having satisfactory ionic conductivity and mechanical properties.
Wu, H. D. et. al., J. Polym. Sci. part B, 1998, 30, 10, 1647; and Wu, H. D. et. al. J. Appl. Polym. Sci., 1998, 69, 6, 1129 disclose a polymer blend, for example PEO modified by novolac type phenolic resin, in which a strong interaction (i.e. H-bonding) between --OH and --O-- shown by the following formula is formed: ##STR1##
The formation of such hydrogen-bonding network is effective not suppressing the crystallinity only, but also improving the mechanical characteristics of the composite material.